Leukocyte adherence inhibition factor (LAIF) was found to be produced in high titers when generated in conical tubes. Both LAIF and leukocyte inhibitory factor (LIF) required the presence of monocytes for their production. LIF production was inhibited by high concentrations (17%) of monocytes but LAIF was still produced under these conditions. Monocytes appeared to act as an accessory cell for antigen presentation in LIF production and there was no detectable requirement for histocompatibility locus-D (HLA-D) compatibility between monocytes and lymphocytes for production of LIF. A modification of the LAI assay was developed to overcome problems of subjectivity and technical variability. This assay used radioisotope labelled mononuclear cells in microtiter plates. Low concentrations of interferon were shown to stimulate production of LIF. Interferon stimulation was antigen-dependent and appeared to be related to more effective or rapid interactions of monocytes with antigen. Interferon may facilitate or accelerate in vitro induction of a primary immune response to these antigens.